A single folded protein is tethered by one end to the AFM tip and by the other to a surface. Pulling the cantilever away stretches the unfolded portion of the chain like a floppy polymer (worm-like chain model). Tension rises steeply as that chain nears its full contour length — until it reaches the unfolding-force threshold of the weakest still-folded domain. That domain snaps open, suddenly adding slack (extra contour length), so the same cantilever position now needs far less tension: force drops abruptly. The cycle repeats, one domain at a time, producing a sawtooth.
F(x) = (kBT/p) · [ 0.25/(1-x/Lc)² − 0.25 + x/Lc ] (worm-like chain)
domain unfolds when F ≥ F_threshold → Lc jumps up → F drops
- Each tooth's peak height is that particular domain's mechanical unfolding force — some folds are simply more stable than others.
- The sawtooth spacing (extension between peaks) tracks the extra contour length released by each domain, typically tens of nanometres.
- Pulling faster raises the apparent unfolding forces slightly (loading-rate / Bell-model effect) — the same domain resists longer under a faster-rising load.
- Once every domain has unfolded, the fully-extended chain stretches until it finally detaches — the last, largest drop in the trace.
- The pull is shown as a slowed-down time-lapse of a real AFM experiment, which normally runs in a fraction of a second — the pulling-speed slider still shows real experimental nm/s values.